The invention relates to novel metal micropowders based on tungsten and/or molybdenum and 3d transition metals.
It is known that an important branch of metallurgy is based on the production of powders which may be used in particular as pigments or in the production of sintered parts.
The metal parts used specifically are generally metal alloys. It will be recalled that metal alloys, depending on the mutual solubility characteristics of the constituent metals, may be single-phase or multiphase systems.
The production of sintered parts using a mixture of pure metal powders gives rise to difficulties when the desire is to obtain a homogeneous sintered part.
It is therefore desirable to prepare prealloyed powders, in which each particle contains the constituent metals of the alloy in the same proportions as the powder in its entirety.
In order to obtain prealloyed powders, it is possible in particular to utilize techniques of coprecipitation of metal hydroxides or salts. The coprecipitates, after drying and, optionally, grinding, are subjected to the action of a reducing agent, for example hydrogen, to give metal powders.
When it is desired to operate starting from water-soluble salts, it is possible to prepare suspensions containing the metal hydroxides or salts in the required proportions and to subject the resulting suspensions to an operation of co-spray drying. This gives particles whose composition in terms of metal hydroxides and/or salts is homogeneous. These particles may be subsequently reduced to prealloyed metal powders with the aid of a reducing agent.
It is known that the techniques of manufacturing metal powders generally lead to agglomerates consisting of a plurality of elementary grains connected to one another at points. Grinding techniques generally enable the number of individual elementary grains to be increased and the number of elementary grains present in the aggregates to be reduced.
As indicated above, the invention relates to micropowders. In the present specification, xe2x80x9cmicropowdersxe2x80x9d are powders such that the largest dimension of the elementary grains is greater than 200 nm and less than or equal to 5 micrometres. The dimensions of the elementary grains may be measured in particular under the scanning electron microscope. The micropowders must be differentiated from nanometric powders, whose elementary grains have dimensions of less than approximately 100 nm.
The invention relates to novel metal powders based on tungsten and/or molybdenum and based on at least one 3d transition metal selected from iron, cobalt, nickel and copper.
The powders of the invention possess advantageous properties in diverse applications, as will be specified in the remainder of the description.
The invention therefore provides a prealloyed metal powder consisting essentially of tungsten and/or molybdenum, at least one transition metal selected from iron, cobalt, nickel and copper, and, optionally, at least one additive, the iron content being less than 50% by weight and the total additive content being less than 3% by weight relative to the total weight of the metals, the said prealloyed metal powder having elementary grain dimensions, measured under the scanning electron microscope, of greater than 200 nm and less than 5 micrometres, with the exception of a powder consisting essentially of tungsten, iron and nickel; and a sintered part obtained with the aid of such a powder.
In the present specification, unless indicated otherwise, a powder xe2x80x9cconsisting essentiallyxe2x80x9d of such and such a metal (xe2x80x9cessentialxe2x80x9d constituents) contains each of these metals in a proportion of more than 1% by weight, and in particular of more than 3% by weight. Such a constituent, when it may be used in a proportion of less than 3%, and in particular of less than 2% or less than 1%, is then considered to be an additive to the alloys in which it is present in such low proportions.
The additives may, in practice, be any metals or metalloids which are capable of improving the properties of the powders or of the sintered parts. Within a given powder, the additives may be selected in particular from all metals which are not essential constituents (as defined above) of the powder, or the oxides of these metals.
The purpose of the presence of additives may in particular be to improve the sintering operations. It is known that the presence of an additive, even in very small amounts (for example, of the order of 0.1%), often makes it possible to reduce considerably the sintering temperature.
The selection of the additives and of their amount may be determined by means of simple, routine experiments.
In the present specification, the percentages of metals are percentages by weight, relative to the total weight of the metals of the powder.
It is known that metal powders have a tendency to undergo oxidation in air, this oxidation increasing over time and with the oxidizability of the metals present. In the powders of the invention, the total oxygen content (measured by reduction using carbon) at the exit from the oven in which the metal hydroxides and/or salts have been reduced is generally less than 2% relative to the total weight of the powder. By optimizing the operating conditions of the hydrogen reduction it is possible, if desired, to obtain markedly lower oxygen contents.
The powders of the invention may be prepared in accordance with the methods of coprecipitation and, optionally, of spray drying, followed by reduction, which have been described above and which are known per se. The selection of the temperature and of the time of reduction may be determined with the aid of simple, routine experiments, in particular by thermogravimetric analysis. It is possible to optimize the size of the elementary grains, in awareness that this size increases with temperature and with the duration of heating, during the reduction operation.
A more detailed description will now be given below of certain classes of powder which form part of the invention. Of course, the invention also relates to sintered parts obtained from such powders, and more generally to any finished industrial articles comprising these powders.
Among the powders of the invention, mention may be made in particular of
(a) those consisting essentially of from 35 to 95% of tungsten and/or molybdenum, from 5 to 65% of copper, and from 0 to 3% of at least one additive; the additives may be selected in particular from iron and cobalt; each additive, if present, may represent, for example, less than 1% of the total weight of the metals of the powder.
Among these powders, mention may be made in particular of those containing from 50% to 95% of tungsten and from 5% to 50% of copper, in particular from 10 to 25% of copper.
Mention may also be made of the powders consisting essentially of from 35 to 95% of molybdenum and from 5 to 65% of copper, and in particular from 10 to 50% of copper.
The powders based on tungsten and/or molybdenum and copper may be used in particular in the production by sintering of electrical contact parts or electrical discharge machining electrodes. In particular, they possess good properties of resistance to the harmful effects of the electric arc (corrosion, oxidation).
Such powders may also be used to produce sintered parts intended for service as heat sinks (or radiators) whose function is to evacuate the heat in certain devices.
(b) those which consist essentially of from 80% to 99.5% by weight of tungsten, from 0.5% to 15% of nickel, from 0 to 10% by weight of iron, from 0 to 10% by weight of copper, and from 0 to 3% of at least one additive.
As an additive it is possible to use, in particular, cobalt, generally in a proportion not greater than 1% by weight relative to the total weight of the metals of the powder. It is also possible to use small proportions (for example from 0 to 1%) of copper, the latter in that case being considered as an additive.
Among these powders, mention may be made in particular of those containing from 90 to 95% by weight of tungsten; and in particular those which consist essentially of tungsten, nickel and iron and which may contain, for example, as additives, from 0 to 1% by weight of cobalt and/or from 0 to 1% by weight of copper.
Among these powders, mention may also be made of those consisting essentially of tungsten, nickel and copper and containing from 0 to 1% of iron and from 0 to 1% by weight of cobalt.
Powders of this kind, of high density, may be used in particular to prepare, by sintering, balance weights, inertial units, tool holders, anti-radiation screens, or else shielding elements.
(c) those consisting essentially of from 5 to 60% by weight of tungsten and/or molybdenum, from 0 to less than 50% by weight of iron, from 0 to 40% by weight of cobalt, from 0 to 80% by weight of nickel, from 0 to 50% by weight of copper, and from 0 to 3% of at least one additive.
Among these powders, mention may be made in particular of those containing from 5% to 60% by weight of tungsten and/or molybdenum, from 20% to 60% by weight of nickel and/or copper, from 20% to less than 50% by weight of iron and from 0 to 30% by weight of cobalt, and in particular those containing from 20 to 60% of nickel and optionally, as additive, from 0 to 1% of copper.
Mention may also be made, among these powders, of those consisting essentially of from 20% to 60% by weight of tungsten and/or molybdenum, from 10% to 40% by weight of cobalt and from 10% to 50% by weight of copper, and optionally iron as additive.
Powders of this kind may be used in particular in the manufacture, by sintering, of diamond tools, in place of the cobalt-based binders conventionally used.
Mention may, finally, be made of powders consisting essentially of from 15 to 40% of copper, from 50 to 80% of nickel and from 5 to 20% of molybdenum.
These powders may be mixed as an additive with iron powder, and optionally with carbon (for example graphite) in order to obtain sintered steels.
(d) those consisting essentially of from 5% to 40% by weight of tungsten and/or molybdenum, from 60% to 95% by weight of nickel, and from 0 to 35% by weight of an additive, the additive being selected for example from cobalt, iron and copper.
Mention may be made in particular of the powders containing from 5 to 30% of tungsten and/or molybdenum and in particular those consisting essentially of tungsten and nickel.
Such powders may be used as binders in the preparation, by sintering, of diamond tools. A binder of this kind improves the mechanical properties of the diamond tools.
(e) those consisting essentially of from 1 to 10% by weight of tungsten and/or molybdenum, from 20 to 80% by weight of cobalt, from 20 to 80% by weight of nickel and from 0 to 35% by weight of an additive.
Among these powders, mention may be made in particular of those consisting essentially of molybdenum, cobalt and nickel, and optionally at least one additive.
Such powders may be used in particular as binders in the preparation, by sintering, of cermets, i.e. abrasive or cutting tools containing titanium carbide.
The invention likewise relates to the use of a powder as defined above in the production of sintered parts which may be used as electrical contacts, heat sinks, electrical discharge machining electrodes, mechanical balancing weights, inertial units, tool holders, anti-radiation screens, shielding elements, sintered steels, or titanium carbide or diamond abrasive or cutting tools. The fields of application of the different categories of powders have been specified above.
Generally speaking, the powders of the invention, used to obtain sintered parts, have the advantage of enhancing the mechanical or physical properties of the parts obtained and/or the advantage of facilitating sintering by making it possible, in particular, to operate at relatively low temperatures and/or pressures and/or of improving the densification of the sintered parts.
The following examples illustrate the invention.